Nonwoven fabrics are widely used in a variety of products. For example, nonwoven fabrics are suitable for use in filters, automotive applications, roofing materials, laminates, composites, backing materials, linings, insulation, medical/surgical applications, bedding, tablecloths and napkins, hygiene and absorbent products. High loft batting nonwoven fabrics are used in a wide variety of products, including comforters, robe wear, and bra cups. Generally nonwoven fabrics are based on polyester, acrylic, nylon, glass and cellulosic fibers which may be bonded with latex adhesives, binder fibers, or polymers in powder form. The bonding of nonwoven fabrics with binder fibers provides a convenient method for making nonwoven fabrics without the need for water-based adhesives which are less environmentally friendly. Nonwoven fabrics bonded with binder fibers are economical to produce, and provide a method for making articles, which are unique or superior in performance. Other applications are uses in yarns to increase strength and reduce pilling or linting, as well as uses in prepregs, preforms and a wide range of composite structures.
Certain copolyesters have proven useful as binder fibers. For example, polyethylene terephthalate (PET) copolyesters containing 1,4-cyclohexanedimethanol having inherent viscosity (I.V.) values in the range of about 0.6 to about 0.8 dl/g have been used in the past as binder fibers to bond polyester or other fibers. Polyesters with lower I.V. values, however, were believed to not have adequate bonding strength.
It is well known that copolyesters can be prepared by processes involving polyesterification and polycondensation. Generally, as described in U.S. Pat. Nos. 2,901,466, 5,017,680, 5,106,944 and 5,668,243, the reactants include glycol components and dicarboxylic acid components. Typically, one dicarboxylic acid component is terephthalic acid and one dihydric alcohol is ethylene glycol. Such copolyesters are relatively inert, hydrophobic materials which are suitable for a wide variety of uses, including, molded articles, such as those used in the automobile and appliance industries, food trays, fibers, sheeting, films and containers, such as bottles. The use of ethylene glycol as the only diol, however, is accompanied by undesirable properties such as yellow discoloration, weak and sometimes brittle fiber binding properties. Indeed, such polymers tend to be opaque, crystalline polymers with high melting temperatures which do make them very suitable for use as binder fibers in many applications. To remedy the problems with polyethylene terephthalates, polyethylene terephthalate polyesters have been formed with 1,4-cyclohexanedimethanol or isophthalic acid.
Previous attempts at forming copolyesters with 1,4-cyclohexanedimethanol have focused upon polyesters having high inherent viscosities, I.V., of greater than 0.6 dl/g, due to the belief that low inherent viscosities would not possess adequate strength. In particular, it was believed that low inherent viscosity polyesters were unable to provide adequate bonding strength to form commercially acceptable binder fibers. Indeed, previous polyethylene terephthalate polyesters containing 1,4-cyclohexanedimethanol were made with inherent viscosities ranging from 0.6 to 0.8 to form binder fibers to bond polyesters or other fibers. However, such attempts have not been entirely successful in providing polyesters having the desired high clarity and hue or bonding capability at low activation temperatures when in the form of a binder fiber.
Other attempts at forming polyesters suitable for use as binder fibers have focused on polyethylene terephthalate copolyesters which have been formed with isophthalic acid and diethylene glycol. Such attempts have resulted in unicomponent and bicomponent binder fibers sold as BELLCOMBI.RTM. available from Unitika of Osaka, Japan, MELTY.RTM. available from Kanebo, Ltd. of Osaka, Japan, CELBOND.RTM. available from KoSa and the like. Previous products however, have failed to recognize the clarity, bonding temperature, bonding strength and cost benefits that can be achieved through polyesters which are the reaction products of dicarboxylic acids and five carbon diols such as 2,2-dimethyl-1,3-propanediol, also referred to as neopentyl glycol (NPG).
There exists a need in the art for cost-effective polyesters, especially polyesters which possess improved clarity and color as well as improved binder fiber bonding strength at low activation temperatures.